Several years ago an antibody was found in the sera of family members of some leukemia patients. This antibody reacts only with red cells of a very rare phenotype. 163 leukemia patients and family members and 165 normal donors sera have been tested. This antibody is found significantly more frequently in the sera of patients with leukemia and their family members as compared with controls (p less than .01). This antibody may define an allele of a specific blood group system thought to be amorphic. In this respect, it will expand the genetic concept of this system.